A thermoresponsive, biodegradable elastomeric material is described herein, along with methods of making the material and uses for the material, particularly uses of the material in repairing defects in heart muscle.
Injectable thermally responsive hydrogels with a lower critical solution temperature (LCST) below body temperature represent promising biomaterials for a variety of biomedical applications, including regional tissue mechanical support as well as drug and cell delivery applications. Generally, the LCST-based phase transition occurs upon warming in situ as a result of entropically-driven dehydration of polymer components, leading to polymer collapse. Various naturally derived and synthetic polymers exhibiting this behavior have been utilized. Natural polymers include elastin-like peptides and polysaccharides derivatives, while notable synthetic polymers include those based on poly(n-isopropyl acrylamide) (pNIPAAm), and amphiphilic block copolymers, often containing poly(ethylene glycol). The structure of pNIPAAm, containing both hydrophilic amide bonds and hydrophobic isopropyl groups, leads to a sharp phase transition at the LCST. Studies suggest that the average number of hydrating water molecules per NIPAAm group falls from 11 to ˜2 upon the hydrophobic collapse above the LCST (32° C.)
pNIPAAm based polymers have been extensively studied as injectable biomaterials for tissue regeneration and drug delivery, yet pNIPAAm itself is a non-biodegradable polymer with a constant LCST of approximately 32° C., which prevents ready clearance from the body at physiologic temperature. This limitation of pNIPAAm has provided the motivation for developing biodegradable NIPAAm-based polymers by conjugating the pNIPAAm with natural biodegradable segments such as MMP-susceptible peptide, gelatin, collagen, hyaluronic acid and dextran. However, these may be only partially bioabsorbable since sufficiently long pNIPAAm segments would remain non-soluble following removal of the natural segments.
Copolymers formed from NIPAAm and monomers with degradable side chains comprise another category of NIPAAm-based bioabsorbable, thermally responsive hydrogels. Hydrolytic removal of hydrophobic side chains increases the hydrophilicity of the copolymer, raising the LCST above body temperature and making the polymer backbone soluble. Due to the relative simplicity of the synthetic process, the most investigated biodegradable monomers have been HEMA-based monomers, such as 2-hydroxyethyl methacrylate-polylactide (HEMA-PLA), 2-hydroxyethyl methacrylate-polycaprolactone (HEMA-PCL) and 2-hydroxyethyl methacrylate-polytrimethylene carbonate (HEMA-PTMC). However, the backbone remnant following hydrolysis, HEMA, presents hydroxyethyl side groups (—CH2CH2—OH), which have a relatively limited effect on remnant polymer hydrophilicity. In previous studies, such hydrogels have been found to be either partially bioabsorbable or completely bioabsorbable, but have required the inclusion of considerably hydrophilic monomers such as acrylic acid (AAc) in the hydrogel synthesis.
Progressive remodeling of the left ventricular (LV) architecture occurs after myocardial infarction (MI). While initially required for maintenance of cardiac output, this response ultimately leads to LV dysfunction and heart failure in the absence of a recurrent ischemic event. Even with current optimal therapy, mortality in end-stage-heart-failure amounts to 20-50% per year. Heart transplantation is applied as the last therapeutic option for patients with terminal heart-failure, but requests for organ transplantation far outstrip the number of donor organs. Therefore, new therapeutic strategies are urgently needed in order to ameliorate both patient prognosis and quality of life.
Following MI, dilatation of the LV cavity has the effect of increasing LV wall tension, which triggers further dilatation of the LV cavity, and progression down a spiral of adverse cardiac remodeling towards the advanced stages of cardiac failure. To restore wall tension, the endoventricular circular patch plasty technique (the Dor procedure) and partial left ventriculectomy (the Batista procedure) have been clinically implemented for severe cardiac dilation and dysfunction many years after an infarction. Employing a similar strategy to limit the remodeling pathway at an earlier stage, epicardial restraint therapies, such as the Acorn Cardiac Support Device, and the Paracor device have been investigated. However, these both apply materials that are non-biodegradable and result in a permanent foreign body encapsulating the epicardium.
Using biodegradable and elastic polyester urethane urea, we recently reported that cardiac patch implantation onto a chronic myocardial infarct prevented further cardiac dilatation and improved contraction, while altering LV wall thickness and compliance. Supported by a finite element model simulation, another concept in locally treating the failing cardiac wall was proposed where a bulking material is injected into the infarcted left ventricular wall to positively alter cardiac mechanics and result in a potentially beneficial reduction of elevated stresses in the infarcted wall. In this numerical model the local systolic fiber stress distribution was determined in an infarcted LV wall injected with a mechanically passive material. The simulation showed that injection of a volume 4.5% that of the total LV wall volume and with a stiffness (elastic modulus) 20% of the natural LV tissue into the infarct border zone could decrease the fiber stress in the border zone of the infarct by 20% compared to a control simulation in which there was no injection. The mechanical simulation also showed that this attenuation effect on LV wall stress increased with the injection volume and the modulus of the injected material.
Thermally responsive hydrogels are particularly attractive materials for injection therapy following MI since it is possible to inject the necessary fluid volumes from a syringe maintained below body temperature. Upon injection and warming hydrogel mechanical properties are increased, the “holding” of the material at the injection site is facilitated and the mechanical benefit of the injected volume on the cardiac wall is increased.
However, despite the advantages of thermally responsive hydrogels known to date, many such gels are too robust, specifically in terms of a prolonged time period of hydrolysis and absorption by native tissue. If hydrolysis does not occur sufficiently rapidly, infiltration by native cells can be slowed, which in turn slows the rate of formation of new tissue at the site of injury.
Accordingly, a need exists, and a substantial challenge remains in the art for versatile biocompatible polymer compounds that can serve as cell growth substrates, for drug delivery purposes and generally for use in patients, for example for cardiac remodeling, where the degradation/hydrolysis time of such compounds can be precisely controlled, and where the polymer compounds and their degradation products exhibit desirable physical properties.